/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.hadoop.util;

/**
 * A PriorityQueue maintains a partial ordering of its elements such that the
 * least element can always be found in constant time. Put()'s and pop()'s
 * require log(size) time.<br/>
 * 用堆来维护一个优先级队列，挺复杂的，有空先看看算法去。
 */
public abstract class PriorityQueue<T> {
	private T[] heap;
	private int size;
	private int maxSize;

	/**
	 * Determines the ordering of objects in this priority queue. Subclasses
	 * must define this one method.
	 */
	protected abstract boolean lessThan(Object a, Object b);

	/** Subclass constructors must call this. */
	@SuppressWarnings("unchecked")
	protected final void initialize(int maxSize) {
		size = 0;
		int heapSize = maxSize + 1;
		heap = (T[]) new Object[heapSize];
		this.maxSize = maxSize;
	}

	/**
	 * Adds an Object to a PriorityQueue in log(size) time. If one tries to add
	 * more objects than maxSize from initialize a RuntimeException
	 * (ArrayIndexOutOfBound) is thrown.
	 */
	public final void put(T element) {
		size++;
		heap[size] = element;
		upHeap();
	}

	/**
	 * Adds element to the PriorityQueue in log(size) time if either the
	 * PriorityQueue is not full, or not lessThan(element, top()).
	 * 
	 * @param element
	 * @return true if element is added, false otherwise.
	 */
	public boolean insert(T element) {
		if (size < maxSize) {
			put(element);
			return true;
		} else if (size > 0 && !lessThan(element, top())) {
			heap[1] = element;
			adjustTop();
			return true;
		} else
			return false;
	}

	/** Returns the least element of the PriorityQueue in constant time. */
	public final T top() {
		if (size > 0)
			return heap[1];
		else
			return null;
	}

	/**
	 * Removes and returns the least element of the PriorityQueue in log(size)
	 * time.
	 */
	public final T pop() {
		if (size > 0) {
			T result = heap[1]; // save first value
			heap[1] = heap[size]; // move last to first
			heap[size] = null; // permit GC of objects
			size--;
			downHeap(); // adjust heap
			return result;
		} else
			return null;
	}

	/**
	 * Should be called when the Object at top changes values. Still log(n)
	 * worst case, but it's at least twice as fast to
	 * 
	 * <pre>
	 * {
	 * 	pq.top().change();
	 * 	pq.adjustTop();
	 * }
	 * </pre>
	 * 
	 * instead of
	 * 
	 * <pre>
	 * {
	 * 	o = pq.pop();
	 * 	o.change();
	 * 	pq.push(o);
	 * }
	 * </pre>
	 */
	public final void adjustTop() {
		downHeap();
	}

	/** Returns the number of elements currently stored in the PriorityQueue. */
	public final int size() {
		return size;
	}

	/** Removes all entries from the PriorityQueue. */
	public final void clear() {
		for (int i = 0; i <= size; i++)
			heap[i] = null;
		size = 0;
	}

	private final void upHeap() {
		int i = size;
		T node = heap[i]; // save bottom node
		int j = i >>> 1;
		while (j > 0 && lessThan(node, heap[j])) {
			heap[i] = heap[j]; // shift parents down
			i = j;
			j = j >>> 1;
		}
		heap[i] = node; // install saved node
	}

	private final void downHeap() {
		int i = 1;
		T node = heap[i]; // save top node
		int j = i << 1; // find smaller child
		int k = j + 1;
		if (k <= size && lessThan(heap[k], heap[j])) {
			j = k;
		}
		while (j <= size && lessThan(heap[j], node)) {
			heap[i] = heap[j]; // shift up child
			i = j;
			j = i << 1;
			k = j + 1;
			if (k <= size && lessThan(heap[k], heap[j])) {
				j = k;
			}
		}
		heap[i] = node; // install saved node
	}
}
